126 MACROMOLECULAR COMPLEXES 



Application of Histochemical and Autoradiography Tech- 

 niques. Thin sections of the "unfixed" and largely undenatured spec- 

 imens can be further subjected to controlled staining, extraction, 

 enzymatic digestion, and numerous other histochemical procedures. 

 The development of adequate high-resolution autoradiography 

 techniques for electron microscopy would be of considerable 

 significance for the field of ultrastructure research, since the 

 structures revealed at the macromolecular level could then be more 

 specifically identified and evaluated in terms of chemical composi- 

 tion and activity. Low-temperature preparation techniques appear 

 to be particularly suitable for this purpose, since the radioactive 

 tracers could be immobilized by rapid freezing in their natural 

 position within the well-preserved tissue matrix ( Fernandez-Moran, 

 1959c ) . The introduction of compounds leading to the formation of 

 silver bromide within the cells during the process of freeze-substi- 

 tution represents one of the most promising approaches. In this 

 high-resolution autoradiography method which is now being de- 

 veloped, the cellular matrix substitutes for the gelatin base of ordi- 

 nary photographic emulsions. Low temperatures markedly reduce 

 the fog and background without impairing the cumulative effects of 

 the radioactivity processes on the silver halides. After exposures 

 of the order of 2 to 8 weeks, the latent image centers are developed 

 and enlarged at low temperatures by using gold thiocyanate ( Hoer- 

 lin and Hamm, 1953) solutions. Interpretation of the images ob- 

 tained with this method is still highly tentative, since the possibility 

 that we might be dealing with "autochemograms" and other spurious 

 effects must first be conclusively ruled out. 



Results 



To establish a reliable basis for evaluation of the fine structures 

 revealed by different types of preparation procedures, the lamellar 

 systems in the nerve myelin sheath and retinal rod outer segments 

 of the frog and guinea pig were examined, in addition to character- 

 istic paracrystalline granules found in the retinal pigment epithe- 

 lium. Using thin sections of standard osmium-fixed and embedded 

 preparations as controls, the effects of freezing, freeze-substitution, 

 staining, and low-temperature polymerization were systematically 

 investigated. 



